Path: utzoo!utgpu!water!watmath!clyde!rutgers!cmcl2!brl-adm!umd5!decuac!c3pe!glenn
From: glenn@c3pe.UUCP (D. Glenn Arthur Jr.)
Newsgroups: sci.electronics
Subject: Re: Library book detectors.
Summary: A few details on the one's I've seen.
Message-ID: <2258@c3pe.UUCP>
Date: 27 Mar 88 08:52:39 GMT
References: <5398@swan.ulowell.edu| <1261@uop.edu| <2521@ihuxv.ATT.COM| <2530@ihuxv.ATT.COM>
Distribution: na
Organization: S4TS
Lines: 47

In article <2530@ihuxv.ATT.COM>, tedk@ihuxv.ATT.COM (Kekatos) writes:
> 
> Well, One system that I happened apon, used a hand-held device which
> was only a power supply with two sharp pins. The foil tag was layered
> in paper and had really bad fake printing on it to make it look a price tag. 
> There was two holes in the paper which exposed the foil. The two sharp 
> pins are matched with the two holes. ZAPPP! The foil is blown away 
> like a fuse. I opened up the paper and foil layers to see. The L-C 
> circuit is destroyed!
> 
> I have not seen any of the library systems  which are described 
> as having "re-useable" tags. But I can think of a few methods of
> designing a re-useable tag. There are magnetic REED switches that
> latch. The tag could be deactived with a magnet. Although this
> sounds to be too simple. Maybe there is a combination of REED
> switches? Then there would be a special magnetic "key" to 
> deactive/active.

I didn't see the beginning of this discussion, but here are my clues.
First, the system described above doesn't sound like systems I've seen
in libraries -- it sounds like it was designed for retail, where a tag
need only be deactivated.

Second, just how small to reed switches get?  I used to work in a library,
and we had foil strips, with glue on both sides, about an eighth of an inch
wide and several inches long.  They were *very* flexible, and although they 
were easily creased, I don't *think* we ever damaged one by creasing it.  
They were activated and deactivated by passing them over an electrical device 
(I think it had a switch to select activation or deactivation), but I have no 
idea just how complex the device was.  As far as one could tell from the 
outside, it could have been merely an electromagnet.

Other systems I have seen have included mainly variations on the physical
dimensions of the tag.  Some are squares maybe an inch on a side and as
thick as a fingernail.  Others are strips, but narrower or wider than the
ones I used to use.  Somebody else in this newsgroup mentioned strips 
about a quarter of an inch wide.

Do the warnings to pacemaker patients posted on the sensing equipment 
constitute a clue?  And has anyone mentioned yet that cardkeys will set
off the detector but that some libraries provide magnetic shields the 
same size as a cardkey which, if placed next to the cardkey, render it
invisible to the sensor?

					D. Glenn Arthur Jr.
-- 
D. Glenn Arthur Jr.	"The Human Vibrator"		..!decuac!c3pe!glenn